The baking of bread, as in many other cases involving convective/conductive heat transfer, is very wasteful of heat energy. In order to bake the bread, it is necessary to first heat the walls of the oven and the air within the oven so that this heat energy may be transferred to the surface of the bread and then slowly carried to the interior via conductive heat transfer. The process is largely limited by the rate at which heat can be conducted from the surface of the bread to the interior. While the process can be speeded up somewhat in a conventional oven through the use of higher ambient temperatures, this is rapidly limited by the temperature at which the surface begins to brown excessively and bakeout becomes too high, as the major negative effects. The result of this is that white bread is conventionally baked for 17 to 23 minutes at 400.degree. F. to 415.degree. F.
Early experiments in baking bread involved the serial application of microwaves and conventional heat, i.e., first applying microwaves to obtain the oven spring and setting the structure and then finish baking with conventional heat to obtain the much needed crust color. While some degree of success was seen, particularly the 915 MHz frequency, the finished loaves left much to be desired since they tended to collapse, had poor rubbery interior structues and very rough unusual crust characteristics. The initial work was done using "Pyrex" and "Corning Ware" type baking dishes, however, there was excessive collapse due to poor sidewall formation. A significant improvement was made using paperboard containers, however, according to the invention a quantum jump in improvement of product quality began with the use of metal baking pans. This is contrary to all accepted practice in microwave processing. Two publications: K. Lorenz et al, "Baking With Microwave Energy", Food Technology, December, 1973; and T. H. Collins, "Exploring the Baking of Bread with Microwaves", FMBRA Bulletin (3), P. 175, 1970; both describe the use of metal baking pans for bread baking in a microwave field as an impossibility since the microwaves will only penetrate the dough through the top and not bake evenly.
The baking of bread is a process which is conventionally a very lengthy operation. Two steps in the process: The proofing of the loaves and the baking of the loaves are especially wasteful of time and energy as done conventionally. Proofing time is limited by the slow march of heat via conductive heat transfer into the center of the loaf. Sine the ambient temperature within the proofer must be kept relatively low, i.e. no more than 135.degree. F. to 140.degree. F. so as to avoid excessive drying of the skin and thermal destruction of the yeast, the driving force for heat transfer is small and proofing times of about one hour for a one pound loaf are common. Baking time is also limited by the ambient conditions in order to avoid excessive darkening of the crust. Baking times of 18 to 22 minutes are common for a one pound loaf. Microwave power, through its unique volume heating means, seemed the ideal way to speed up both these processes and this especially so since we discovered a means of heating the dough in metal pans in the microwave oven.
While our experimentation was initially directed at using microwave power for the entire proofing process and thereby shortening it to one tenth or less than the conventional proofing time, we found that this was not possible. What was discovered, however, was that if the dough was first partially proofed by conventional means, then microwave power could be use to finish the proofing process in a significantly shorter total proofing time.